Should I download my BHP magazines by one or two rounds to insure reliability? The answer is "maybe." If you have an older Hi Power with the humped feed ramp, this may be necessary if using other than ball ammunition. The tapered case of the 9mm tends to let the round point lower and lower in the magazine as the number of rounds increases and while this was not a problem with FMJ in the older guns, it did cause problems when JHP ammo became widely available. I cannot prove it, but believe that Mec-Gar was not the OEM magazine supplier back then and the magazines that I have from that time period do not hold the top round as high as do current BHP Mec-Gar or "Browning" magazines. I say that because using new Mec-Gar magazines in older HPs with the humped ramp very often feed fine. Likewise, the newer Hi Powers tend to feed JHP fine with the older magazines. If you have a Mk II or Mk III, it's been my experience that you can load to full capacity with about any JHP and suffer no loss in reliability. One exception can be Corbon 115 and 125-grain JHP, so check those. They usually work fine, but check them in your pistol to be sure.
If interested, here's Mec-Gar's home page: http://www.mec-gar.com/index.html
I also highly recommend the following site for detailed information on Browning Hi Power magazines: